This invention relates to an improved on-car brake lathe apparatus. More specifically, this invention relates to an apparatus and method for automatically compensating for the lateral runout of a lathe apparatus with respect to a vehicle hub. The invention further includes a novel runout measurement and control system that describes the runout of a disc brake assembly and directs a corrective signal to an automated control system for adjustment in order to effectively compensate of lateral runout. The novel runout apparatus and method may also be advantageously utilized in other practical applications in order to align two concentrically attached rotating shafts.
A brake system is one of the primary safety features in every road vehicle. The ability to quickly decelerate and bring a vehicle to a controlled stop is always critical to the safety of the vehicle occupants and those in the immediate vicinity. In this, a vehicle braking system is designed and manufactured to exacting specifications and rigorous inspection.
One of the main components of a brake system are the disc brake assemblies typically mounted on the front wheels of most passenger vehicles. Generally, the disc brake assemblies include a caliper (cooperating with a brake hydraulic system), brake pads, a hub, and a rotor. The caliper supports and positions a pair of brake pads on opposing sides of a brake rotor. In a hubless brake rotor (i.e. when the rotor and hub are separate components), the rotor is secured to the vehicle hub, via a rotor hat, with a series of bolts for rotation with the hub about a vehicle spindle axis. When a vehicle driver depresses a brake pedal thereby activating the hydraulic system, the brake pads are forced together and toward the rotor to grip the friction surfaces of the rotor.
Disc brake assemblies must be maintained to manufacturers specifications throughout the life of the vehicle in order to assure optimum performance and maximum safety. However, several problems have plagued the automotive industry since the inception of disc brakes.
A significant problem in brake systems is usually referred to as “lateral runout.” Generally, lateral runout is the side-to-side movement of the friction surfaces of the rotor as it rotates with the vehicle hub about a spindle axis. Referring to FIG. 1, for example, there is shown a rotor having friction surfaces on its lateral sides. A rotor is mounted on a vehicle hub for rotation about the horizontal spindle axis X. In an optimum rotor configuration, the rotor is mounted to rotate in a plane Y that is precisely perpendicular to the spindle axis X. Generally, good braking performance is dependant upon the rotor friction surfaces being perpendicular to the spindle's axis of rotation X and parallel to one another (“parallelism”). In the optimum configuration, the opposing brake pads will contact the friction surfaces of the rotor at perfect 90 degree angles and will exert equal pressure on the rotor as it rotates. More typically, however, the disc brake assembly will produce at least a degree of lateral runout that deviates from the ideal configuration. For example, a rotor will often rotate in a canted plane Y′ and about an axis X′ which is a few thousandths of an inch out of axial alignment with the spindle (shown in exaggerated fashion in FIG. 1). In this rotor configuration, the brake pads, which are perpendicular to the spindle axis X, will not contact the friction surfaces of the rotor along a constant pressure plane.
The lateral runout of a rotor is the lateral distance that the rotor deviates from the ideal plane of rotation Y during a rotation cycle of the rotor. A certain amount of lateral runout is inherently present in the hub and rotor assembly. This lateral runout often results from defects in individual components. For example, rotor friction surface runout results when the rotor friction surfaces are not perpendicular to the rotor's own axis of rotation, rotor hat runout results when the hat connection contains deviations that produce an off center mount, and stacked runout results when the runouts of the components are added or “stacked” with each other. An excessive amount of lateral runout in a component or in the assembly (i.e. stacked runout) will generally result in brake noise, pedal pulsation, and a significant reduction in overall brake system efficiency. Moreover, brake pad wear is uneven and accelerated with the presence of lateral runout. Typically, manufacturers specify a maximum lateral runout for the friction surfaces, rotor hat, and hub that is acceptable for safe and reliable operation.
After extended use, a brake rotor must be resurfaced in order to bring the brake assembly within manufacturers specifications. This resurfacing is typically accomplished through a grinding or cutting operation. Several prior art brake lathes have been used to resurface brake rotors. These prior art lathes can be categorized into three general classes: (1) bench mounted lathes; (2) on-car caliper-mounted lathes; and (3) on-car hub mounted lathes. As discussed below, the on-car hub mounted lathes have proven to be the most reliable and accurate rotor refinishing lathes in the industry.
Bench mounted lathes, for example, that disclosed in U.S. Pat. No. 3,540,165 to Lanham, are inefficient and do not have rotor matching capabilities. In order to resurface a rotor on a bench mounted lathe, the operator is first required to completely remove the rotor from the hub assembly. The operator then mounts the rotor on the bench lathe using a series of cones or adapters. After the cutting operation, the operator remounts the rotor on the vehicle spindle. Even if the rotor is mounted to the lathe in a perfectly centered and runout free manner, runout between the rotor and hub is not accounted for in the bench lathe resurfacing operation. In addition, bench lathes are susceptible to bent shafts which introduce runout into a machined rotor. This runout is then carried back to the brake assembly where it may be added with hub runout to produced a stacked runout effect.
Similarly, caliper-mounted lathes, for example, that disclosed in U.S. Pat. No. 4,388,846 to Kopecko et al., have had limited success in compensating for lateral runout, but require time consuming manual operations. During a rotor refinishing procedure, the brake caliper must first be removed in order to expose the rotor and hub. Once removed, the caliper mounting bracket is freed and can be used to mount an on-car caliper-mount lathe. The caliper-mount lathes are wholly unacceptable for many reasons including the lack of a “rigid loop” connection between the driving motor and cutting tool and the inability to assure a perpendicular relationship between the cutting tools and the rotor. Moreover, the caliper-mount lathes do not have any reliable means for measuring and correcting lateral runout. Typically and in much the same manner as described below with reference to the hub mounted lathes, a dial indicator is utilized in determining the total amount of lateral runout in the disc assembly. This measurement technique is problematic in terms of time, accuracy and ability of automechanics to comfortably use the equipment.
On-car hub mounted lathes, for example, that disclosed in U.S. Pat. No. 4,226,146 to Ekman, assigned to the assignee of the instant application, and incorporated by reference into the disclosure herein, have proven to be the most accurate and efficient means for resurfacing the rotor.
Referring now to FIG. 2, there is shown an Ekman type on-car disc brake lathe 10 for mounting to the hub of a vehicle 14. The lathe 10 includes a body 16, driving motor 18, adapter 20, and cutting assembly 22. The lathe is also provided with a stand and anti-rotation post (not shown), either of which can be used to counter the rotation of the lathe during a resurfacing operation. After the brake caliper is removed, the adapter 20 is secured to the hub of the vehicle 14 by using the wheel lug nuts. The lathe body 16 is then mounted to the adapter 20.
At this point in the prior art procedure, the operator must determine the total amount of lateral runout and make an appropriate adjustment. Specifically, the operator first mounts a dial indicator 26 to the cutting head 22 using a knob 28. The dial indicator 26 is positioned to contact the vehicle 14 at one of its distal ends as shown in FIG. 2. Once the gauge 26 is properly positioned, the operator is required to take the following steps in order to measure and compensate for lateral runout:                (1) The operator mates the lathe to the rotor using the adapter and procedure outlined above.        (2) The operator activates the lathe motor 18 which causes the adapter 20, and thereby the brake assembly hub and rotor, to rotate. The total lateral runout of the assembly will be reflected by corresponding lateral movement in the lathe body.        (3) The lateral movement of the lathe-body is then quantified by using the gauge 26. Specifically, the operator observes the dial indicator to determine the high and low deflection points and the corresponding location of these points on the lathe.        (4) Upon identifying the highest deflection of the dial indicator, the operator “bumps” the motor and stops the rotation at the point of the identified highest deflection.        (5) The operator then makes an adjustment to compensate for runout of the assembly. This is accomplished by careful turning of the adjustment screws 24. Specifically, there are four adjustment screws and the correct screw(s) must be turned depending on the location of the high point. The affect of turning the screws is to adjust the orientation of the lathe body with respect to the adapter 20 (and therefore the rotor and hub) to mechanically compensate for the runout of the assembly. The operator adjusts the screws until the highest deflection point is reduced by half as determined by reference to the dial indicator 26.        (6) The operator activates the lathe motor 18 and observes the dial indicator 26 to again identify the highest deflection of the dial. If the maximum lateral movement of the lathe body, as measured by the needle deflection, is acceptable (i.e. typically less than 3/1000) then mechanical compensation is complete and the lathe turning operation can commence. Otherwise, further measurement and adjustment will be necessary by repeating steps (1) to (6).        
The cutting operation is then performed by adjusting the tool holder 22 and cutting tools 23, and setting the proper cutting depth.
Although the hub mounted on-car brake lathe was a considerable advance in the disc brake lathe industry, its structure and corresponding procedure for compensating for lateral runout of the disc brake assembly has practical limitations.
First, as readily apparent, upon observation of steps (1)-(6) above, the Ekman procedure requires a significant amount of time to determine and adjust for lateral runout of the brake assembly. Although the specific amount of time necessary will vary based upon operator experience, the procedure time for even the most trained and experience is significant and can substantially increase the cost associated with rotor refinishing to the vehicle owner and the shop. Second, the prior art system and procedure requires the shop owner and technicians to undergo extensive education and operator training in order to assure that proper mechanical compensation for lateral runout is accomplished. Moreover, the Ekman system is operator specific. That is, the accuracy and success of measurement and adjustment of lateral runout will vary from operator to operator.
In general, the prior art systems and procedures are problematic with respect to accuracy in the measuring and adjusting of lateral runout. The prior art systems require an operator to locate a high reading for lateral runout by viewing the gauge 26; often, the operator is required to “bump” the motor to relocate the high point once it has been identified. Moreover, even if the operator correctly locates and/or relocates the high point of lateral runout, human errors are often introduced during the adjustment process. For example, selecting the correct screw or screws 24 and applying the precise amount of torque necessary for adjustment is often difficult and imprecise.
The difficulties and limitations suggested in the preceding are not intended to be exhaustive, but rather are among many which demonstrate that although significant attention has been devoted to disc brake lathes, such systems will admit to worthwhile improvement.